Fiber Bragg gratings (hereinafter “FBG”) have been widely used in optical telecommunication systems, fiber lasers, as well as in the sensing industry. Wavelength stabilizer of pump laser and optical filters are only examples of the well-known applications of FBG in optical telecommunication.
FBGs can generally be written on any type of optical fibers, be it single mode, multi-mode, polarization maintaining (hereinafter “PM”) and so on. In a nutshell, PM fibers have the particularity that they will maintain the polarization of the transmitted light beam along all the length of the fiber. However, FBGs of PM fiber have an added complexity compared to FBGs written on standard single mode fiber in term of manufacturing and measurements, especially the latter. This added complexity comes from the birefringence properties of the PM fiber, meaning that the refractive index of the PM fiber is slightly greater in its slow axis than in its fast axis. Therefore, the optical properties of a FBG written on a PM fiber will vary depending on whether the light is coupled in the slow axis, the fast axis or in a combination thereof. Hence, depending on the measurement conditions (e.g. the coupling of the light), the measured properties will vary. For example, the center wavelength reflected by a FBG of a PM fiber measured in the slow axis will be greater than the center wavelength reflected in the fast axis.
Therefore, when it is necessary to measure the optical properties of a FBG, or of another optical device, using a PM fiber, it is necessary to measure the state of polarization (hereinafter “SOP”). In other words, the light power ratio between the fast axis and the slow axis must be determined and monitored during optical measurements of FBG (and other optical devices) written on a PM fiber.
In a perfect situation, the ideal test set-up for this purpose would be to have an all-PM configuration. Indeed, the perfect set-up would comprises devices and/or components that are all polarization maintaining such as a PM light source, a PM coupler, a PM circulator and so on. However, for an all-PM set-up, the problem is not only unreasonable cost, it is also availability. Indeed, in some cases, some of the aforementioned elements simply do not exist.
There are, in the prior art, methods to measure optical properties of optical devices or components using reference FBG. In fact, the term “reference fiber Bragg grating” was first introduced by Martin in his article entitled “Use of a Sampled Bragg Grating as an In-Fiber Optical Resonator for the Realization of a Referencing Optical Frequency Scale for WDM Communications” (OFC' 1997 Technical Digest, pp. 284-285). However, Martin's publications, related to “reference fiber Bragg grating”, define its application as an in-fiber optical wavelength/frequency reference only.
In 1998, Miller et al. (U.S. Pat. No. 5,838,437) proposed a fixed fiber Fabry-Perot (FFP) filter used in combination with a reference FBG as wavelength and frequency reference.
In 2000, Davis et al. (U.S. Pat. Nos. 6,118,914 and 6,403,949) disclosed the use of a temperature stabilized FBG as a wavelength reference.
In 2003, Valente et al. (U.S. Pat. No. 6,658,171) proposed a technology called “Optical Fiber Bragg Grating Polarizer”. In their invention, the FBG was used as a component to polarize a light wave.
Finally, in 2004, Peupelmann et al. (U.S. Pat. No. 6,816,260) proposed a device referred to as a fiber polarimeter which utilized oblique FBG to couple out of a light wave, the coupling result depending on the SOP. Therefore, the portions of the coupled light could be used to determine the four Stokes parameters. However, in Peupelmann's invention, the FBG was used as a tool to couple out a portion of a light wave not as a reference. Furthermore, their invention is not based on FBG of PM fiber.
The problem with all these references and methods is that they do not provide a simple yet effective method to accurately measure the optical properties of a FBG in a PM fiber according to a predetermined SOP. There is therefore a need for such a new method.